cpu,soc/x86: always include cpu/x86/mtrr on x86 CPUs/SoCs

All x86-based CPUs and SoCs in the coreboot tree end up including the
Makefile in cpu/x86/mtrr, so include this directly in the Makefile in
cpu/x86 to add it for all x86 CPUs/SoCs. In the unlikely case that a new
x86 CPU/SoC will be added, a CPU_X86_MTRR Kconfig option that is
selected be default could be added and the new CPU/SoC without MTRR
support can override this option that then will be used in the Makefile
to guard adding the Makefile from the cpu/x86/mtrr sub-directory.

In cpu/intel all models except model 2065X and 206AX are selcted by a
socket and rely on the socket's Makefile.inc to add x86/mtrr to the
subdirs, so those models don't add x86/mtrr themselves. The Intel
Broadwell SoC selects CPU_INTEL_HASWELL and which added x86/mtrr to the
subdirs. The Intel Xeon SP SoC directory contains two sub-folders for
different versions or generations which both add x86/mtrr to the subdirs
in their Makefiles.

Change-Id: I743eaac99a85a5c712241ba48a320243c5a51f76
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44230
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/soc/intel/quark/Makefile.inc

@@ -3,7 +3,6 @@
 ifeq ($(CONFIG_SOC_INTEL_QUARK),y)
 
 subdirs-y += romstage
-subdirs-y += ../../../cpu/x86/mtrr
 
 bootblock-y += bootblock/esram_init.S
 bootblock-y += bootblock/bootblock.c